1. Field of the Invention
The present invention relates to a polarization insensitive coherent lightwave detector and, more particularly, to a coherent lightwave detection system where only the polarization state of the local oscillator must be known to recover information from the transmitted optical signal. The polarization state of the transmitted signal is irrelevant to achieving recovery.
2. Description of the Prior Art
Coherent lightwave detection systems are becoming increasingly prevalent in many lightwave communication arrangements. In many such systems, conventional heterodyne/homodyne techniques are used to recover the transmitted signal. These techniques require that the polarization state of the local oscillator be accurately aligned to the polarization state of the message signal to assure accurate recovery of the message. This limitation is considered to be a serious drawback to the advance of coherent lightwave detectors. One prior art arrangement which addresses this polarization problem is described in the article entitled "Demodulation of Optical DPSK Using In-Phase and Quadrature Detection" by T. G. Hodgkinson et al. appearing in Electronic Letters, Vol. 21, No. 19, September 1985, at pp. 867-77, In this arrangement, a 90.degree. optical hybrid is used to achieve the in-phase and quadrature detection, where the local oscillator is connector to a first input of the hybrid via a first polarization controlling member and the transmitted DPSK signal is connected to the remaining input of the hybrid via a second polarization controlling member. However, since the polarization state of both signals are subject to drift, the polarization controllers must be monitored to insure correct operation of the demodulator.
An alternative coherent detection system, described in U.S. Pat. No. 4,506,388 issued to M. Monerie et al on Mar. 19, 1985, although capable of being used with a message signal of random polarization, still requires that the polarization state of the local oscillator match that of the message signal.
A need remains in the prior art, therefore, for a coherent detection system which is truly independent of the polarization state of the transmitted message signal.